Vibrating device and piezoelectric pump

ABSTRACT

A piezoelectric pump includes a piezoelectric element, an intermediate plate, and a vibrating plate. The piezoelectric element has a substantially flat plate shape. The intermediate plate is bonded to a principal surface of the piezoelectric element and applies a residual stress in a compressive direction to the piezoelectric element. The vibrating plate is bonded to the intermediate plate such that the vibrating plate faces a principal surface of the piezoelectric element and receives a residual stress in a compressive direction from the intermediate plate. In addition, the vibrating plate defines a portion of a wall surface of a pump chamber having an open hole. A fluid passage is provided in the piezoelectric pump. The fluid passage communicates with the outside of the chamber at one end thereof, and communicates with the pump chamber through the open hole at the other end.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a vibrating device including apiezoelectric portion, such as a piezoelectric element, and moreparticularly, to a piezoelectric pump which conveys fluid by causing avibrating plate which defines a wall surface of a pump chamber tovibrate using the piezoelectric portion.

2. Description of the Related Art

In general, a piezoelectric thin film is sensitive to tensile stress andeasily breaks when a tensile stress is applied thereto. Accordingly, atechnique to prevent the breakage of the piezoelectric thin film due tothe tensile stress by applying a residual stress in a compressivedirection is known (refer to, for example, Japanese Unexamined PatentApplication Publication No. 2004-146640).

The piezoelectric thin film is formed on a substrate having a highercoefficient of linear expansion than that of the piezoelectric thin filmin a heated environment. Then, when the piezoelectric thin film iscooled, the substrate contracts by a greater amount than the amount bywhich the piezoelectric thin film contracts. As a result, a residualstress in the compressive direction is applied to the piezoelectric thinfilm.

FIGS. 1A and 1B are sectional views of a piezoelectric thin film or apiezoelectric element in a known structure.

A structure 200 is obtained by thermally bonding a piezoelectric layer201 to a substrate 202 and performing a cooling process after thethermal bonding process. FIG. 1A is a sectional view of the structure200 in a static state after the cooling process. When the structure 200is in this state, a residual stress in a tensile direction is applied tothe substrate 202 and a residual stress in a compressive direction isapplied to the piezoelectric layer 201. In this structure 200, since theresidual stresses applied to the piezoelectric layer and the substrateare different from each other, the piezoelectric layer expands and thesubstrate contracts in the structure 200. As a result, the structure 200slightly warps. Due to the expansion of the piezoelectric layer, thereis a large residual stress gradient in the piezoelectric layer 201. Theresidual stress in the compressive direction is extremely small in anarea near the surface of the piezoelectric layer 201.

The above-described piezoelectric element or the piezoelectric thin filmmay be used in a vibrating device or a piezoelectric pump which vibratesthe piezoelectric element or the piezoelectric thin film by applying analternating voltage thereto. In such a case, when the piezoelectriclayer vibrates, the piezoelectric layer receives a tensile stress or acompressive stress.

FIG. 1B is a sectional view of the structure 200 in the state in whichthe structure 200 is bent to be convex on the piezoelectric-layer sidedue to the vibration. In the structure 200, since the residual stress inthe compressive direction is applied to the piezoelectric layer 201 inadvance, the tensile stress applied due to the vibration is reduced.However, since the piezoelectric layer 201 is initially warped and theresidual stress in the compressive direction is extremely small in thearea near the surface of the piezoelectric body, there is a risk that acrack will be formed in the surface the piezoelectric body and thepiezoelectric body will break.

In addition, in a piezoelectric pump in which a wall surface of a pumpchamber is formed by a vibrating plate and the vibrating plate isvibrated by the piezoelectric element or the piezoelectric thin film toconvey fluid in the pump chamber, there is a risk that the fluid cannotbe conveyed if the piezoelectric layer breaks.

SUMMARY OF THE INVENTION

To overcome the problems described above, preferred embodiments of thepresent invention provide a vibrating device which prevents apiezoelectric layer from being warped and which reduces the risk thatthe piezoelectric layer will be broken by a tensile stress applied tothe piezoelectric layer due to vibration, and a piezoelectric pump whichreduces the risk that fluid cannot be conveyed.

A vibrating device according to a preferred embodiment of the presentinvention includes a piezoelectric portion, a first bonding portion, anda second bonding portion. The piezoelectric portion preferably has asubstantially flat plate shape. The first bonding portion is preferablybonded to a principal surface of the piezoelectric portion and applies aresidual stress in a compressive direction to the piezoelectric portion.The second bonding portion is preferably bonded to the first bondingportion such that the second bonding portion faces the piezoelectricportion, and receives a residual stress in the compressive directionfrom the first bonding portion. Alternatively, the second bondingportion is bonded to the piezoelectric portion such that the secondbonding portion faces the first bonding portion, and applies a residualstress in the compressive direction to a bonding surface of thepiezoelectric portion.

Thus, the residual stress in the compressive direction is applied to thepiezoelectric portion. In the structure including the piezoelectricbody, the first bonding portion, and the second bonding portion,residual stresses remain in a balanced state at both principal surfacesof the structure, so that warping of the structure is prevented.Therefore, the residual stress in the compressive direction is appliedto a portion of the piezoelectric portion in an area near the surfacethereof. Even when an alternating voltage is applied to thepiezoelectric portion to cause vibration, the tensile stress applied tothe piezoelectric portion due to the vibration can be canceled by theresidual stress in the compressive direction which remains in thepiezoelectric portion.

A piezoelectric pump according to a preferred embodiment of the presentinvention includes the above-described vibrating device and a pump body.A fluid passage is provided in the pump body, and the fluid passageextends from a pump chamber and communicates with the outside through anopen hole. Preferably, the first bonding portion or the second bondingportion functions as a vibrating plate which defines a portion of a wallsurface of the pump chamber. In such a case, the risk that the fluidcannot be conveyed due to breakage of the piezoelectric portion can bereduced.

A piezoelectric pump according to another preferred embodiment of thepresent invention includes a piezoelectric element, an intermediateplate, a vibrating plate, and a pump body. The piezoelectric elementpreferably has a substantially flat plate shape. The intermediate plateis bonded to a principal surface of the piezoelectric element andapplies a residual stress in a compressive direction to thepiezoelectric element. The vibrating plate is bonded to the intermediateplate such that the vibrating plate faces a principal surface of thepiezoelectric element and receives a residual stress in the compressivedirection from the intermediate plate. In addition, the vibrating platedefines a portion of a wall surface of a pump chamber having an openhole. A fluid passage is provided in the pump body. The fluid passageextends from the pump chamber and communicates with the outside throughthe open hole.

If the piezoelectric element, the vibrating plate, and the intermediateplate are bonded together in a heated environment, a compressive stressis applied to the piezoelectric element and the vibrating plate and atensile stress is applied to the intermediate plate after a coolingprocess. Therefore, the residual stresses remain in the piezoelectricelement and the vibrating plate in a balanced manner, and warping can beprevented after the cooling process. Consequently, the residual stressin the compressive direction that remains is applied to a portion of thepiezoelectric portion in an area near the surface thereof. Therefore,even when an alternating voltage is applied to the piezoelectric portionto cause vibration, the tensile stress applied to the piezoelectricportion due to the vibration can be canceled by the residual stress inthe compressive direction which remains in the piezoelectric portion.

A coefficient of linear expansion of the intermediate plate maypreferably be greater than a coefficient of linear expansion of thevibrating plate and a coefficient of linear expansion of thepiezoelectric element. In such a case, the residual stress in thecompressive direction remains in a balanced manner in the area near thesurface of the piezoelectric portion, and formation of cracks in thesurface of the piezoelectric portion can be effectively prevented.

Preferably, a plurality of components of the pump body have the same orsubstantially the same coefficient of linear expansion, so thatdeformation of the components of the pump body due to temperaturevariation can be prevented. In such a case, a piezoelectric pump inwhich deformation due to temperature variation is small can be obtained.

In the structure including the piezoelectric element, the intermediateplate, and the vibrating plate, if the overall coefficient of linearexpansion of the structure is less than the coefficient of linearexpansion of the pump body, a residual stress in the compressivedirection remains in the vibrating plate. In such a case, even if thevibrating plate is deformed due to temperature variation, no tensilestress is applied to the vibrating plate. Therefore, a piezoelectricpump in which deformation due to temperature variation is small can beobtained. In addition, leakage of vibration to the pump body does notreadily occur and the pump efficiency increases.

According to various preferred embodiments of the present invention, theresidual stresses remain in a balanced state so that warping of thestructure can be prevented. As a result, a large residual stress in thecompressive direction remains in the piezoelectric body. Therefore, evenif a tensile stress is applied to the piezoelectric layer due to thevibration, the risk that the piezoelectric layer will break can begreatly reduced.

Other elements, features, steps, characteristics and advantages of thepresent invention will become more apparent from the following detaileddescription of the preferred embodiments with reference to the attacheddrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B are diagrams illustrating a known structure.

FIGS. 2A and 2B show sectional views of a piezoelectric pump accordingto a first preferred embodiment of the present invention.

FIG. 3 is a plan view of a top plate included in the piezoelectric pump.

FIG. 4 is a plan view of a flow-path plate included in the piezoelectricpump.

FIG. 5 is a plan view of a pump-chamber top plate included in thepiezoelectric pump.

FIG. 6 is a plan view of a pump-chamber forming plate included in thepiezoelectric pump.

FIGS. 7A and 7B are diagrams illustrating a diaphragm structure includedin the piezoelectric pump.

FIG. 8 is a plan view of a bottom plate included in the piezoelectricpump.

FIGS. 9A to 9E are diagrams illustrating the operation of thepiezoelectric pump.

FIGS. 10A and 10B show sectional views of the diaphragm structure.

FIGS. 11A to 11C are diagrams illustrating other examples of a diaphragmstructure of the piezoelectric pump.

FIGS. 12A and 12B are diagrams illustrating the diaphragm structure.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A first preferred embodiment of the present invention in which avibrating device according to a preferred embodiment of the presentinvention is included in a piezoelectric pump will be described.

FIG. 2A is a sectional view of a piezoelectric pump 100 according to thefirst preferred embodiment. FIG. 2B is a diagram illustrating the mannerin which fluid flows through the piezoelectric pump 100. In thispreferred embodiment, an example of the structure of a piezoelectricpump used for air-cooling in an electronic device is described.

The piezoelectric pump 100 preferably includes a top plate 10, aflow-path plate 20, a pump-chamber top plate 30, a pump-chamber formingplate 40, a vibrating plate 51, an intermediate plate 53, apiezoelectric element 54, and a bottom plate 60. The vibrating plate 51,the intermediate plate 53, and the piezoelectric element 54 arethermally bonded together in advance preferably by applyingthermosetting adhesive and heating the adhesive. Thus, a diaphragmstructure 50, which will be described below, is provided. A portion ofthe piezoelectric pump 100 excluding the diaphragm structure 50 definesa pump body.

The top plate 10 includes an open hole 11, and is bonded to theflow-path plate 20. A plan view of the top plate 10 is shown in FIG. 3.The top plate 10 is preferably a flat or substantially flat plate havinga rectangular or substantially rectangular external shape, and ispreferably made of a rigid material, such as a cold rolled steel plate(SPCC), for example. The open hole 11 is provided at a central portionof the top plate 10. The open hole 11 communicates with the outside ofthe piezoelectric pump 100 and a flow-path central chamber 21 providedin the flow-path plate 20.

The flow-path plate 20 is provided with the flow-path central chamber 21and fluid passages 22, and is bonded to the top plate 10 and thepump-chamber top plate 30. A plan view of the flow-path plate 20 isshown in FIG. 4. The flow-path plate 20 is preferably a flat orsubstantially flat plate which has the same or substantially the sameexternal shape as that of the top plate 10, and is preferably made ofthe same material as that of the top plate 10. The flow-path centralchamber 21 is provided at a central portion of the flow-path plate 20,and four fluid passages 22 are provided so as to extend from theflow-path central chamber 21 toward the four corners of the flat plate.The flow-path central chamber 21 has a diameter that is greater than thediameter of the open hole 11, and communicates with the open hole 11 andan open hole 31 provided in the pump-chamber top plate 30. The fluidpassages 22 communicate with the flow-path central chamber 21 and inlets32 provided in the pump-chamber top plate 30. Since a plurality of fluidpassages 22 communicate with the flow-path central chamber 21, theresistance at the fluid passages 22 is reduced and the gas is attractedto the flow-path central chamber 21. Therefore, the gas flow rate can beincreased.

The pump-chamber top plate 30 is provided with the open hole 31 andinlets 32, and is bonded to the flow-path plate 20 and the pump-chamberforming plate 40. A plan view of the pump-chamber top plate 30 is shownin FIG. 5. The pump-chamber top plate 30 is preferably a flat orsubstantially flat plate which has the same or substantially the sameexternal shape as that of the flow-path plate 20, and is preferably madeof the same material as that of the flow-path plate 20. The open hole 31is provided at a central portion of the pump-chamber top plate 30, andfour inlets 32 are provided at the four corners of the flat plate. Theopen hole 31 and the open hole 11 have substantially the same diameter,and communicate with the flow-path central chamber 21 and a pump chamber41 provided in the pump-chamber forming plate 40. In addition, theinlets 32 communicate with the fluid passages 22 and inlets 42 providedin the pump-chamber forming plate 40. Although the open hole 31 and theopen hole 11 may have different diameters, the diameters are preferablyless than the diameter of the flow-path central chamber 21. In addition,the pump-chamber top plate 30 may be made of a material different fromthat of the flow-path plate 20. For example, the pump-chamber top plate30 may preferably be made of a material having spring elasticity.

The pump-chamber forming plate 40 is provided with the pump chamber 41and the inlets 42, and is bonded to the pump-chamber top plate 30 andthe vibrating plate 51. A plan view of the pump-chamber forming plate 40is shown in FIG. 6. The pump-chamber forming plate 40 is preferably aflat or substantially flat plate which has the same or substantially thesame external shape as that of the pump-chamber top plate 30, and ispreferably made of the same material as that of the pump-chamber topplate 30. The pump chamber 41, which has a circular or substantiallycircular shape, is provided at a central portion of the pump-chamberforming plate 40, and four inlets 42 are provided at the four corners ofthe flat plate. The pump chamber 41 has a diameter that is greater thanthat of the flow-path central chamber 21, and communicates with the openhole 31. In addition, the inlets 42 communicate with the inlets 32 andinlets 52 provided in the vibrating plate 51.

The intermediate plate 53 and the piezoelectric element 54 are bonded tothe vibrating plate 51 in advance. More specifically, they are adheredtogether preferably by applying an adhesive and curing the adhesive byapplying heat. The diaphragm structure 50 is defined by theabove-mentioned components. A sectional view of the diaphragm structure50 is shown in FIG. 7A, and a plan view of the diaphragm structure 50 isshown in FIG. 7B. The vibrating plate 51 is preferably a flat orsubstantially flat plate having the same or substantially the sameexternal shape as that of the pump-chamber forming plate 40, and fourinlets 52 are provided at the four corners of the flat plate. Thevibrating plate 51 is bonded to the pump-chamber forming plate 40 andthe bottom plate 60. The inlets 52 communicate with the inlets 42 andinlets 62 provided in the bottom plate 60. The intermediate plate 53 ispreferably a flat or substantially flat plate having a circular externalshape, and is thermally bonded to the vibrating plate 51 at a centralportion thereof. The piezoelectric element 54 is preferably a flat orsubstantially flat plate having the same or substantially the sameexternal shape as that of the intermediate plate 53, and is thermallybonded to the intermediate plate 53 at a central portion thereof.

The bottom plate 60 is provided with a piezoelectric-element receivingsection 61 and the inlets 62, and is bonded to the vibrating plate 51 inthe diaphragm structure 50. A plan view of the bottom plate 60 is shownin FIG. 8. The bottom plate 60 is preferably a flat or substantiallyflat plate having the same or substantially the same external shape asthat of the vibrating plate 51. The piezoelectric-element receivingsection 61, which preferably has a circular or substantially circularshape, is provided at a central portion of the bottom plate 60, and fourinlets 62 are provided at the four corners of the flat plate. Thepiezoelectric-element receiving section 61 accommodates the intermediateplate 53 and the piezoelectric element 54 in the diaphragm structure 50.The inlets 62 communicate with the inlets 52. The thickness of thebottom plate 60 is preferably greater than the sum of the thickness ofthe intermediate plate 53, the thickness of the piezoelectric element54, and the amount of deformation of the piezoelectric element 54.Accordingly, the piezoelectric element 54 is prevented from coming intocontact with a mounting board when the piezoelectric pump 100 is mountedto the surface of the mounting board.

The diaphragm structure 50 has a unimorph structure in which thepiezoelectric element 54 is provided only on one side of the vibratingplate 51. The piezoelectric element 54 vibrates in an expansionvibration mode when an alternating voltage (sine wave or square wave) isapplied to the piezoelectric element 54. Accordingly, the overall bodyof the diaphragm structure 50 is bent in a thickness direction thereof.In addition, the pump-chamber top plate 30 resonates in accordance withthe bending of the diaphragm structure 50. As a result, the distancebetween the vibrating plate 51 and the pump-chamber top plate 30 changesand gas flows in and out of the pump chamber 41 through the open hole31.

The alternating voltage is applied to the piezoelectric element 54 at afirst-order resonance frequency or a third-order resonance frequency ofthe diaphragm structure 50. In this case, a volume variation caused bydisplacement components can be significantly increased as compared tothe case in which an alternating voltage is applied at a frequency otherthan the first-order resonance frequency or the third-order resonancefrequency. As a result, the flow rate can be greatly increased.

FIGS. 9A to 9E show sectional views of the main portion of thepiezoelectric pump 100 illustrating the operation of the piezoelectricpump 100. In FIGS. 9A to 9E, the arrows show the flow of gas.

FIG. 9A shows the main portion in the state in which the alternatingvoltage is not applied to the piezoelectric element. In this state, thevibrating plate 51 is substantially flat.

FIG. 9B shows the main portion in the state after the first quarter of aperiod of the alternating voltage. At this time, the vibrating plate 51is bent so as to be downwardly convex. Therefore, the distance betweenthe vibrating plate 51 and the open hole 31 in the pump chamber 41increases so that the gas is sucked into the pump chamber 41 from thefluid passages 22 through the open hole 31.

FIG. 9C shows the main portion in the state after the next quarter ofthe period. At this time, the vibrating plate 51 becomes flat again andthe distance between the vibrating plate 51 and the open hole 31 in thepump chamber 41 decreases. Therefore, the gas in the pump chamber 41 ispushed out through the open holes 31 and 11. The gas in the fluidpassages 22 is caused to flow together with the gas which flows outthrough the open hole 11, and the external gas which surrounds the openhole 11 is also caused to flow together with the gas which flows outthrough the open hole 11.

FIG. 9D shows the main portion in the state after the next quarter ofthe period. At this time, the vibrating plate 51 is bent to be upwardlyconvex, and the distance between the vibrating plate 51 and the openhole 31 in the pump chamber 41 further decreases. Therefore, the gas inthe pump chamber 41 is pushed out through the open holes 31 and 11. Thegas in the fluid passages 22 is caused to flow together with the gaswhich flows out through the open hole 11, and the external gas whichsurrounds the open hole 11 is also caused to flow together with the gaswhich flows out through the open hole 11.

FIG. 9E shows the main portion in the state after the next quarter ofthe period. At this time, the vibrating plate 51 becomes flat again andthe distance between the vibrating plate 51 and the open hole 31 in thepump chamber 41 increases. Since the volume increases, a portion of thegas which flows through the fluid passages 22 is sucked into the pumpchamber 41 through the open hole 31. However, most of the gas whichflows through the fluid passages 22 continuously flows out through theopen hole 11 due to inertia thereof.

The above-described deformation of the vibrating plate 51 isperiodically repeated. When the distance between the vibrating plate 51and the open hole 31 in the pump chamber 41 increases, the gas is causedto flow into the pump chamber 41 through the flow-path central chamber21, the fluid passages 22, the inlets 32, the inlets 42, the inlets 52,and the inlets 62. When the distance between the vibrating plate 51 andthe open hole 31 in the pump chamber 41 decreases, the gas flows out ofthe piezoelectric pump through the pump chamber 41, the open hole 31,the flow-path central chamber 21, and the open hole 11. Thepiezoelectric element is vibrated at a high frequency so that the gascontinuously flows out through the open hole 11 without stopping due tothe inertia of the gas that flows through the fluid passages 22.

An example of the diaphragm structure 50 will now be described belowwith reference to FIG. 10.

The thickness of the piezoelectric element 54 is preferably about 0.2mm, for example, and the coefficient of linear expansion thereof in anormal temperature (about 20° C. to about 30° C.) is preferably about1×10⁻⁶/K, for example. The piezoelectric element 54 vibrates when analternating voltage is applied thereto. Preferably, the frequency of thealternating voltage is about 23 kHz and the amplitude of the alternatingvoltage is about ±5V (10 Vpp) to ±10 V (20 Vpp), for example.

The vibrating plate 51 is preferably made of Fe-42Ni alloy, for example.The thickness of the vibrating plate 51 is preferably about 0.08 mm, andthe coefficient of linear expansion thereof in a normal temperature ispreferably about 7×10⁻⁶/K, for example.

The intermediate plate 53 is preferably made of SPCC, for example. Thethickness of the intermediate plate 53 is preferably about 0.15 mm, andthe coefficient of linear expansion thereof in a normal temperature ispreferably about 11×10⁻⁶/K, for example, which is greater than thecoefficient of linear expansion of the piezoelectric element 54 and thecoefficient of linear expansion of the vibrating plate 51.

The diaphragm structure 50 is preferably formed by bonding thepiezoelectric element 54, the intermediate plate 53, and the vibratingplate 51 together by applying thermosetting adhesive and curing theadhesive by applying heat, for example. Therefore, the piezoelectricelement 54, the intermediate plate 53, and the vibrating plate 51 arebonded together in a state in which each of them is thermally expanded.

The coefficient of linear expansion of the intermediate plate 53 isgreater than the coefficient of linear expansion of the piezoelectricelement 54 and the coefficient of linear expansion of the vibratingplate 51. Therefore, in a cooling process, the intermediate plate 53tries to contract by a greater amount than an amount by which thepiezoelectric element 54 and the vibrating plate 51 contract. Therefore,a residual stress in a compressive direction remains in each of thepiezoelectric element 54 and the vibrating plate 51 and a residualstress in a tensile direction remains in the intermediate plate 53 afterthe cooling process.

FIG. 10A is a partial sectional view of the diaphragm structure 50 in astatic state.

When the diaphragm structure 50 is in this state, the residual stress inthe tensile direction is applied to the intermediate plate 53. Inaddition, the residual stress in the compressive direction is applied tothe piezoelectric element 54 and the vibrating plate 51. The residualstress in the compressive direction is applied at both sides of theintermediate plate 53, that is, in both the piezoelectric element 54 andthe vibrating plate 51. Therefore, the residual stress remains in abalanced state. As a result, warping of the diaphragm structure 50 isprevented. Since warping of the diaphragm structure 50 is prevented, alarge residual stress in the compressive direction also remains in anarea near the surface of the piezoelectric element 54. Preferably, thecoefficient of linear expansion of the piezoelectric element 54 and thecoefficient of linear expansion of the vibrating plate 51 are close toeach other, so that warping of the diaphragm structure 50 can beeffectively prevented.

FIG. 10B is a partial sectional view of the diaphragm structure 50 inthe state in which the diaphragm structure 50 is bent to be convex onthe piezoelectric element side due to the application of the alternatingvoltage.

The diaphragm structure 50 vibrates when an alternating voltage isapplied to the piezoelectric element 54 included in the diaphragmstructure 50. Accordingly, a tensile stress or a compressive stress isapplied to the piezoelectric element 54. In this diaphragm structure 50,warping of the diaphragm structure 50 does not significantly occurbefore the application of the alternating voltage. Therefore, a largeresidual stress in the compressive direction is applied to thepiezoelectric element 54. Consequently, the tensile stress applied tothe piezoelectric element 54 when it is bent can be canceled by theresidual stress in the compressive direction, and the risk that thepiezoelectric body will break can be reduced.

The heating temperature is set in accordance with the glass transitiontemperature of the thermosetting resin, and is preferably in the rangeof about 80° C. to about 150° C., for example. If the heatingtemperature is too high, there is a possibility that the bonding portionwill become separated due to the thermal shock.

The coefficient of linear expansion of the vibrating plate 51 in thenormal temperature is preferably in the range of about 1×10⁻⁶/K to about7×10⁻⁶/K, for example, and the vibrating plate 51 may also be made ofFe-36Ni alloy or other suitable material instead of Fe-42Ni alloy. Thecoefficient of linear expansion of the intermediate plate 53 in thenormal temperature is preferably in the range of about 11×10⁻⁶/K toabout 17×10⁻⁶/K, for example, and the intermediate plate 53 may also bemade of SUS430, SUS304, or other suitable material, instead of SPCC. Thecoefficient of linear expansion of the piezoelectric element 54 in thenormal temperature is preferably in the range of about 1×10⁻⁶/K to about7×10⁻⁶/K, for example, and the piezoelectric element 54 may also be madeof PT or other suitable material other than PZT. If the coefficient oflinear expansion of the intermediate plate 53 is excessively high ascompared to those of the piezoelectric element 54 and the vibratingplate 51, there is a risk that the bonding portion will become separateddue to the thermal shock.

The ratio of the thickness of the vibrating plate 51 to that of thepiezoelectric element 54 is preferably in the range of about 0.4 toabout 0.6, for example. If the thickness of the vibrating plate 51 istoo small, the residual stress gradient in the piezoelectric element 54will be excessively large. In such a case, sufficient residual stress inthe compressive direction cannot be applied to the surface of thepiezoelectric element 54. If the thickness of the vibrating plate 51 istoo large, sufficient vibration amplitude cannot be obtained.

The ratio of the thickness of the intermediate plate 53 to that of thepiezoelectric element is preferably in the range of about 0.5 to about1.0, for example. If the thickness of the intermediate plate 53 is toosmall, sufficient residual stress in the compressive direction cannot beapplied to the piezoelectric element 54, and the resonance frequency ofthe vibration of the diaphragm structure 50 decreases. If the thicknessof the intermediate plate 53 is too large, the degree of resonance ofthe bending vibration of the diaphragm structure 50 becomes too high andsufficient amplitude cannot be obtained.

Preferably, components other than the diaphragm structure 50 are made ofthe same material. In such a case, deformation of the components of thepiezoelectric pump 100 other than the diaphragm structure 50 can beprevented, and the thermal characteristic of the piezoelectric pump 100can be reduced.

The coefficient of linear expansion of the overall body of the diaphragmstructure 50 is preferably set to be less than the coefficient of linearexpansion of the components other than the diaphragm structure 50. Forexample, the components of the piezoelectric pump 100 other than thediaphragm structure 500 may be made of the same material as that of theintermediate plate 53, so that the coefficient of linear expansion ofthe components is equivalent to that of the intermediate plate 53. Insuch a case, when the components are adhered to the diaphragm structure50 and are heated, the components expand by a greater amount as comparedto the amount by which the diaphragm structure 50 expands. Therefore, aresidual stress in the compressive direction is applied to the vibratingplate 51 of the diaphragm structure 50. In such a case, even if thevibrating plate is deformed due to temperature variation, no tensilestress is applied to the vibrating plate. Therefore, a piezoelectricpump having a small temperature characteristic can be obtained.

Next, diaphragm structures according to other preferred embodiments ofthe present invention that can be used in the piezoelectric pump will bedescribed. FIGS. 11A to 11C show sectional views of the diaphragmstructures.

FIG. 11A shows a diaphragm structure 70 including a vibrating plate 71,an upper piezoelectric element 74A, an intermediate plate 73, and alower piezoelectric element 74B. The upper piezoelectric element 74A,the intermediate plate 73, and the lower piezoelectric element 74Bpreferably have the same or substantially the same shape, and the upperpiezoelectric element 74A and the lower piezoelectric element 74B arepreferably made of the same material. The intermediate plate 73 ispreferably made of a material having a high coefficient of linearexpansion, and the upper piezoelectric element 74A and the lowerpiezoelectric element 74B are preferably made of a material having a lowcoefficient of linear expansion. The vibrating plate 71 is preferablymade of a material having a coefficient of linear expansion that issimilar to that of the upper piezoelectric element 74A.

The vibrating plate 71, the upper piezoelectric element 74A, theintermediate plate 73, and the lower piezoelectric element 74B areadhered to each other with thermosetting adhesive and are thermallybonded together by curing the adhesive by applying heat.

Also in this structure, a residual stress in a tensile direction isapplied to the intermediate plate 73 and a residual stress in acompressive direction is applied to the upper piezoelectric element 74Aand the lower piezoelectric element 74B in the diaphragm structure 70.The residual stress in the compressive direction is applied at bothsides of the intermediate plate 73, that is, in both the upperpiezoelectric element 74A and the lower piezoelectric element 74B.Therefore, the residual stress remains in a balanced state. As a result,warping of the diaphragm structure 70 is prevented, and a large residualstress in the compressive direction also remains in areas near thesurfaces of the piezoelectric elements 74A and 74B.

FIG. 11B shows a diaphragm structure 90 including a vibrating plate 91,a piezoelectric element 94, and a bonding portion 95. The piezoelectricelement 94 and the bonding portion 95 preferably have the same orsubstantially the same shape. The bonding portion 95 is preferably madeof, for example, Invar. The vibrating plate 91 is preferably made of amaterial having a high coefficient of linear expansion, and thepiezoelectric element 94 and the bonding portion 95 are preferably madeof a material having a low coefficient of linear expansion.

The vibrating plate 91, the piezoelectric element 94, and the bondingportion 95 are adhered to each other with thermosetting adhesive and arethermally bonded together by curing the adhesive by applying heat.

Also in this structure, a residual stress in a tensile direction isapplied to the vibrating plate 91 and a residual stress in a compressivedirection is applied to the piezoelectric element 94 and the bondingportion 95 in the diaphragm structure 90. The residual stress in thecompressive direction is applied at both sides of the vibrating plate91, that is, in both the piezoelectric element 94 and the bondingportion 95. Therefore, the residual stress remains in a balanced manner.As a result, warping of the diaphragm structure 90 is prevented, and alarge residual stress in the compressive direction remains in an areanear the surface of the piezoelectric element 94 and an area near thesurface of the bonding portion 95.

FIG. 11C shows a diaphragm structure 110 including a vibrating plate111, a piezoelectric element 114, and a bonding portion 115. Thepiezoelectric element 114 and the bonding portion 115 preferably havethe same or substantially shape. The bonding portion 115 and thevibrating plate 111 are preferably made of the same material. Thevibrating plate 111 and the bonding portion 115 are preferably made of amaterial having a high coefficient of linear expansion, and thepiezoelectric element 114 is preferably made of a material having a lowcoefficient of linear expansion.

The vibrating plate 111, the piezoelectric element 114, and the bondingportion 115 preferably are adhered to each other with thermosettingadhesive and are thermally bonded together by curing the adhesive byapplying heat.

The coefficient of linear expansion of the vibrating plate 111 and thecoefficient of linear expansion of the bonding portion 115 are greaterthan the coefficient of linear expansion of the piezoelectric element114. Therefore, the vibrating plate 111 and the bonding portion 115 tryto contract by a greater amount than an amount by which thepiezoelectric element 114 contracts in a cooling process. Therefore,after the cooling process, a residual stress in a compressive directionis applied to the piezoelectric element 114 and a residual stress in atensile direction is applied to the vibrating plate 111 and the bondingportion 115. The piezoelectric element 114 has a bimorph structureobtained by adhering two piezoelectric plates to each other.

FIG. 12A is a partial sectional view of the diaphragm structure 110 in astatic state.

When the diaphragm structure 110 is in this state, the residual stressin the tensile direction is applied to the vibrating plate 111 and thebonding portion 115, and the residual stress in the compressivedirection is applied to the piezoelectric element 114. The residualstress in the tensile direction is applied at both sides of thepiezoelectric element 114, that is, in both the vibrating plate 111 andthe bonding portion 115. Therefore, the residual stress remains in abalanced state. As a result, warping of the diaphragm structure 110 isprevented. Since warping of the diaphragm structure 110 is prevented, alarge residual stress in the compressive direction also remains in thepiezoelectric element 114. Preferably, the coefficients of linearexpansion of the bonding portion 115 and the vibrating plate 111 areclose to each other, so that warping of the diaphragm structure 110 canbe effectively prevented.

FIG. 12B is a partial sectional view of the diaphragm structure 110 inthe state in which the diaphragm structure 110 is bent due to thealternating voltage applied thereto.

The diaphragm structure 110 vibrates when an alternating voltage isapplied to the piezoelectric element 114 included in the diaphragmstructure 110. Accordingly, a tensile stress or a compressive stress isapplied to the piezoelectric element 114. In this diaphragm structure110, warping of the diaphragm structure 110 does not readily occurbefore the application of the alternating voltage. Therefore, acompressive stress with a small gradient in the thickness direction ofthe piezoelectric element 54 remains in the piezoelectric element 54.Consequently, the tensile stress applied to the piezoelectric element114 when it is bent can be canceled by the residual stress in thecompressive direction, and the risk that the piezoelectric body willbreak can be reduced.

Although a piezoelectric pump used for air-cooling in an electronicdevice is described above, preferred embodiments of the presentinvention may also be applied to, for example, a pump for conveying gas,such as oxygen, to a fuel cell. Alternatively, preferred embodiments ofthe present invention may also be applied to a cooling-water conveyingpump or a fuel conveying pump which conveys liquid. The structure of thepiezoelectric pump is also not limited to the above-described structure,and the present invention may be applied to any type of piezoelectricpump as long as a wall surface of a pump chamber is defined by avibrating plate included in a diaphragm structure.

While preferred embodiments of the present invention have been describedabove, it is to be understood that variations and modifications will beapparent to those skilled in the art without departing the scope andspirit of the present invention. The scope of the present invention,therefore, is to be determined solely by the following claims.

What is claimed is:
 1. A piezoelectric pump comprising: a vibratingdevice comprising: a piezoelectric portion including a substantiallyflat plate having a circular or substantially circular external shape; afirst bonding portion fixed to a principal surface of the piezoelectricportion, the first bonding portion being arranged to apply a residualstress in a compressive direction to a bonding surface of thepiezoelectric portion; and a second bonding portion bonded to the firstbonding portion such that the second bonding portion faces thepiezoelectric portion, the second bonding portion being arranged toreceive a residual stress in the compressive direction from the firstbonding portion; and a pump body including a pump chamber and a fluidpassage provided therein, the fluid passage extending from the pumpchamber and communicating with the outside through a first open holeprovided in the pump body; wherein the first bonding portion includes avibrating plate which defines a portion of a wall surface of the pumpchamber; the vibrating plate extends to an outer periphery of the pumpbody; and the pump body further includes a pump chamber top plate whichdefines another wall surface of the pump chamber, is opposed to thevibrating plate, and includes a second open hole therein whichcommunicates with the fluid passage and the first open hole.
 2. Apiezoelectric pump comprising: a vibrating device comprising: apiezoelectric portion including a substantially flat plate having acircular or substantially circular external shape; a first bondingportion fixed to a principal surface of the piezoelectric portion, thefirst bonding portion being arranged to apply a residual stress in acompressive direction to a bonding surface of the piezoelectric portion;and a second bonding portion bonded to the first bonding portion suchthat the second bonding portion faces the piezoelectric portion, thesecond bonding portion being arranged to receive a residual stress inthe compressive direction from the first bonding portion; and a pumpbody including a pump chamber and a fluid passage provided therein, thefluid passage extending from the pump chamber and communicating with theoutside through a first open hole provided in the pump body; wherein thesecond bonding portion includes a vibrating plate which defines aportion of a wall surface of the pump chamber; the vibrating plateextends to an outer periphery of the pump body; and the pump bodyfurther includes a pump chamber top plate which defines another wallsurface of the pump chamber, is opposed to the vibrating plate, andincludes a second open hole therein which communicates with the fluidpassage and the first open hole.
 3. A piezoelectric pump comprising: avibrating device comprising: a piezoelectric portion including asubstantially flat plate having a circular or substantially circularexternal shape; a first bonding portion fixed to a principal surface ofthe piezoelectric portion, the first bonding portion being arranged toapply a residual stress in a compressive direction to a bonding surfaceof the piezoelectric portion; and a second bonding portion bonded to thepiezoelectric portion such that the second bonding portion faces thefirst bonding portion, the second bonding portion being arranged toapply a residual stress in the compressive direction to a bondingsurface of the piezoelectric portion; and a pump body including a pumpchamber and fluid passage provided therein, the fluid passage extendingfrom the pump chamber and communicating with the outside through a firstopen hole provided in the pump body; wherein the first bonding portionincludes a vibrating plate which defines a portion of a wall surface ofthe pump chamber; the vibrating plate extends to an outer periphery ofthe pump body; and the pump body further includes a pump chamber topplate which defines another wall surface of the pump chamber, is opposedto the vibrating plate, and includes a second open hole therein whichcommunicates with the fluid passage and the first open hole.
 4. Apiezoelectric pump comprising: a vibrating device comprising: apiezoelectric portion including a substantially flat plate having acircular or substantially circular external shape; a first bondingportion fixed to a principal surface of the piezoelectric portion, thefirst bonding portion being arranged to apply a residual stress in acompressive direction to a bonding surface of the piezoelectric portion;and a second bonding portion bonded to the piezoelectric portion suchthat the second bonding portion faces the first bonding portion, thesecond bonding portion being arranged to apply a residual stress in thecompressive direction to a bonding surface of the piezoelectric portion;and a pump body including a pump chamber and a fluid passage providedtherein, the fluid passage extending from the pump chamber andcommunicating with the outside through a first open hole provided in thepump body; wherein the second bonding portion includes a vibrating platewhich defines a portion of a wall surface of the pump chamber; thevibrating plate extends to an outer periphery of the pump body; and thepump body further includes a pump chamber top plate which definesanother wall surface of the pump chamber, is opposed to the vibratingplate, and includes a second open hole therein which communicates withthe fluid passage and the first open hole.
 5. A piezoelectric pumpcomprising: a piezoelectric portion including a substantially flat platehaving a circular or substantially circular external shape; anintermediate plate fixed to a principal surface of the piezoelectricportion, the intermediate plate being arranged to apply a residualstress in a compressive direction to the piezoelectric portion; avibrating plate fixed to the intermediate plate such that the vibratingplate faces the principal surface of the piezoelectric portion, thevibrating plate being arranged to receive a residual stress in thecompressive direction from the intermediate plate and to define aportion of a wall surface of a pump chamber; a pump body including thepump chamber and a fluid passage provided therein, the fluid passageextending from the pump chamber and communicating with the outsidethrough a first open hole provided in the pump body; wherein thevibrating plate extends to an outer periphery of the pump body; and thepump body further includes a pump chamber top plate which definesanother wall surface of the pump chamber, is opposed to the vibratingplate, and includes a second open hole therein which communicates withthe fluid passage and the first open hole.
 6. The piezoelectric pumpaccording to claim 5, wherein a coefficient of linear expansion of theintermediate plate is greater than a coefficient of linear expansion ofthe vibrating plate and a coefficient of linear expansion of thepiezoelectric portion.
 7. The piezoelectric pump according to claim 6,wherein the pump body includes a plurality of components which arestacked together and which have substantially the same coefficient oflinear expansion.
 8. The piezoelectric pump according to claim 7,wherein the piezoelectric portion, the intermediate plate, and thevibrating plate define a structure having an overall coefficient oflinear expansion that is less than a coefficient of linear expansion ofthe pump body.